Advances in polymerization and catalysis have resulted in the capability to produce a plethora of new polymers having improved physical and chemical properties. These new polymers have proved useful in producing a wide variety of superior products with new applications. With the development of new catalysts, the variety of polymerization types (solution, slurry, high pressure, or gas phase) available for producing a particular polymer has also been greatly expanded. Advances in polymerization technology have thus resulted in more efficient and highly productive processes which have proved economically advantageous. Illustrative of these advances is the development of technology utilizing supported catalyst systems for the heterogeneous catalysis of olefin polymerizations. These heterogeneous catalyst systems provide a surface area for catalysis, and are typically prepared by deposition of reactive catalytic moieties on a particulate support, usually an inorganic oxide. There are a variety of different methods described in the art for supporting catalytic moieties on supports for use in slurry or gas phase polymerization processes.
For example, heterogeneous catalysts, such as supported Ziegler-Natta or chromium-based catalysts, have had significant impact on the polyolefin industry. Supported Ziegler-Natta catalysts afford high activity and high stereo-regular content in olefin polymerization while supported chromium-based catalyst systems typically produce polyolefins with a narrow molecular weight distribution and high molecular weight. Coupled with advantages over homogeneous catalysts, such as increased thermal stability, ease of separation, and no solvent limitations, heterogeneous catalyst systems may be industrially advantageous. Such catalyst systems opened scientific floodgates, leading to an explosion of new chemistry, new processes, and new products in the polyolefin industry.
However, as with the advent of any new technology, new challenges are presented with the heterogenization of catalyst systems. For example, heterogeneous catalyst systems are theoretically less efficient than their homogeneous counterparts because the polymerization reaction must necessarily take place on the surface of the catalyst. Any catalyst moiety not present at the surface may remain unused, whereas all the molecules in a homogeneous catalyst are theoretically available. Further, heterogeneous catalyst systems tend to be more sensitive to poisoning, such as by soft ligands and the oxygen content and humidity of air, than homogeneous catalysts. As such, heterogeneous catalyst systems are usually handled under highly inert, dry, and oxygen free atmospheres. Yet further, circulation of heterogeneous catalyst systems through the polymerization system poses unique problems. Whereas homogeneous catalysts are usually introduced as a solution in a wet feed, heterogeneous catalyst systems may be introduced as a dry feed. The use of a dry catalyst feed presents several advantages over a wet feed, such as ease of handling and conservation of solvent. Thus, the dry feed of a supported catalyst system may provide both economic and environmental advantages to a polyolefin manufacturer.
In a dry catalyst feed process, it is desirable that the heterogeneous catalyst system flow freely through catalyst feeders and feed lines. A number of catalyst feed systems for gas phase reactors are known to those skilled in the art. Well known systems include systems including a catalyst storage vessel connected to a feed chamber which is in turn connected to a gas phase reactor. The catalyst storage vessel, feed chamber, and gas phase reactor may be connected to each other by filling and emptying valves. Typically, a heterogeneous catalyst system is conveyed from the storage vessel through the catalyst feed system to the reactor by maintaining the reactor at a pressure lower than that in the catalyst feed system. Valves incorporated in such systems allow a given quantity of heterogeneous catalyst system to move from the storage vessel to a feed chamber or metering device and then to the reactor.
U.S. Pat. No. 4,162,894 describes a pressure equalized feed system incorporating a ball check feed valve and a downstream positive shut-off valve for controlling intermittent feed of heterogeneous catalyst system. U.S. Pat. No. 4,687,381 describes a feed system using a shut-off valve and metering device for periodic delivery of powdered heterogeneous catalyst system. These, and other conventional catalyst feed systems, however, may not overcome the problems caused by poor catalyst flow.
The flow properties of a heterogeneous catalyst system are dependent on a multitude of factors, such as the nature of the catalytic moiety and the nature of the support, among other factors. For example, heterogeneous catalyst systems, including a carboxylate metal salt in conjunction with a supported metallocene catalyst system such as described in U.S. Pat. Nos. 6,306,984 and 6,300,436 substantially improve process operability, but have sticky or statically inclined particulate flow. Pre-polymerized catalysts, such as described in U.S. Pat. No. 4,579,836, made by treating the catalyst with a small amount of monomer under polymerization conditions, may demonstrate improved catalyst particle strength and product characteristics, but may have a concomitant degradation in catalyst flow properties.
Heterogeneous catalyst systems with poor flow are more difficult to feed to a reactor, and may prevent the desired smooth and continuous introduction of catalyst into the reactor. Poorly flowing heterogeneous catalyst systems may also stick to walls of catalyst feed vessels, feeders, and feed lines, causing buildup and possible clogging of feed lines. Buildup of residual catalyst system in feed lines interferes with control of delivery, and ultimately process control. Further, poorly flowing heterogeneous catalyst systems may impair accurate delivery of the desired catalyst system amount to the polymerization reactor. This may result in poor polymerization efficiency and low production. Ineffective catalyst delivery, therefore, affects system performance, stability, and, ultimately, the polymer product.
The myriad of problems caused by poorly flowing catalyst systems significantly impairs process operability and efficiency, in some cases to the point of reactor shutdown. Where the buildup proceeds to the extent that feed lines are clogged, the polymerization system may have to be taken off line to clean the clogged lines. Frequent repair and/or replacement of system valves and clogged lines can prove time-consuming and expensive. Repair of clogged lines and valves results in reactor downtime, increased personnel hours, and replacement of parts, all of which add to the cost of the process, and may result in a significant economic loss to the polymer manufacturer.
Solutions to poorly flowing dry heterogeneous catalyst systems have been addressed by modifying the method of preparation of the catalyst system. For example, the catalyst system components may be combined in a particular order; the ratio of the various catalyst system components may be manipulated; the contact time and/or temperature when combining the components while forming a catalyst system may be varied; or additional compounds may be added to the catalyst system. Examples of these include: WO 96/11961 discusses an antistatic agent as a component of a supported catalyst system; U.S. Pat. Nos. 5,332,706 and 5,473,028 disclose incipient impregnation as a particular technique for forming a catalyst system; U.S. Pat. Nos. 5,427,991 and 5,643,847 describe the chemical bonding of non-coordinating anionic activators to supports; U.S. Pat. No. 5,492,975 discusses polymer bound metallocene catalyst systems; and U.S. Pat. No. 6,680,276 discusses a composition of a carboxylate metal salt in combination with a heated polymerization catalyst system to improve the catalyst system flow and operability of the catalyst system.
Other solutions have been directed towards the catalyst feed system. For example, U.S. Pat. No. 5,433,924 is directed towards using filters positioned to vent the fill chamber. The positioning of the filter provides a pressure differential to encourage poorly flowing catalyst system to flow through the feed lines, and provides a means for removing and recycling residual catalyst system. U.S. Pat. No. 4,690,804 describes the use of a ball check feed valve for the transfer of polymer coated catalyst system.
Some of the techniques discussed above for remedying poor catalyst system flow may affect catalyst system productivity, catalyst system activity, may not work for particular catalyst system types, may add significant cost to the catalyst system manufacture process, and may cause additional problems such as sheeting and fouling during the polymerization process. Accordingly, there exists a need for methods, processes, and systems to improve catalyst system flow.